We are going to explore electricity and electronics.
GOALS (This week will be a continuation of week 4):
· Learn to use an oscilloscope.
Up until now it has been sufficient to measure voltages with digital meters. The meters display the voltage difference between the two probes. The next circuit we will build will produce a changing voltage. We will use a special IC (integrated circuit), the 555 timer, to produce an oscillating voltage. One can drive a speaker with an oscillating voltage to produce a note. In building and testing the circuit we need to be able to measure the voltage as it changes. The numbers on the display of a meter a far too difficult to interpret. These meters also typically respond very slowly so they cannot follow the changes in a moderately fast oscillation.
A more effective way to examine changing voltage is to look at a picture that displays the changes. A good way to think about this is to imagine how an earthquake or an EKG is recorded. Basically a pen is placed on a moving piece of paper. The pen moves up and down as the signal increases and decrease while the paper is pulled along under the pen the result is a chart or graph of the signal as time progresses. This method is illustrated in the drawing below.
You can of course carry out such a scheme yourself. If you move your hand up and down while pulling a piece of paper sideways you will draw a curve. If you were to move your hand up as voltage increases and down when the voltage decreases you would be drawing a picture that would display the voltage as time progresses. The oscilloscope is basically doing the electronic equivalent and drawing a picture for us of the voltage versus time. A screen replaces the paper in the model. The oscilloscope reuses the screen when it reaches the end so that several drawings are superimposed on the screen at one time. You can simulate that by returning your paper to the starting point and then moving the paper through a second and third time. What you discover is that the image cam get obscured as more and more drawings overlap. To avoid this problem the oscilloscope is equipped with a trigger. The trigger synchronizes the pen and the paper motion so that if the same basic shape is repeatedly being drawn the second, third and all subsequent drawings fall exactly on top of each other.
To control the scales and the triggering several adjustments need to be made. Our goal will be to be able to attach the oscilloscope to a source of changing voltage and measure the voltage by setting the scales and trigger settings. It is far easier to demonstrate how to use the oscilloscope in the lab so the details will be covered during class.
One signal that we will examine in the lab is a sinusoidal oscillation. A plot of voltage versus time for a sinusoidal voltage is shown below.
The time between two peaks is referred to as the period of the oscillation.
frequency of oscillation = 1/period
Our measurements of the voltage from a AC outlet will show that the AC power that we use in our homes has this type of behaviour with a frequency of 60 cycles per second.
In order to receive a grade of A you must successfully complete the lab portion of the course, read prelab material, and pass the lab quiz. In order to pass you must get 75% of the questions correct.
In order to receive a grade of B you must successfully complete the lab portion of the course. The quiz is optional. For fun feel free to try the quiz.
You are not allowed to consult with other students while taking the quiz. You will be given a limited time period to complete the quiz. You may refer to any written material.
You may take the quiz at any time during the week but only once.
The quiz will be available starting on the day you cover the material and ending one week later.
If you have troubles please discuss the difficulties with DRG.
You are not allowed to consult with other students while taking the quiz. The quiz will be available at least from lab until following lab. You will be given 30 minutes to complete the quiz and you may refer to any written material.